Analog amplifiers suffer from several disadvantages which have given rise to the development of switching, or discrete state, amplifiers. For a given power output, especially at relatively high amplitude levels, analog amplifiers suffer from poor efficiency, primarily because of the need to bias the active elements into linear regions for amplification without distortion, as in well-known Class A and Class B analog amplifiers. Since the power dissipated in the active elements is substantial, the resulting amplifier power efficiency is poor.
Moreover, the low efficiency of analog amplifiers increases the heat generated therein and results in size and weight penalties caused by the need to remove the heat generated. At relatively high power levels, for example, the amplifiers may require a large passive mechanical heat exchanger and/or the use of forced air heat exchangers for heat removal purposes.
Low efficiency also implies the need for larger power supplies for a given power output which further aggravates the efficiency problem because of the heat losses in the power supplies themselves, thereby imposing in effect a double penalty. Lower efficiency also increases costs because the power handling elements are larger, components having wider temperature ranges of operation are required, and heat removal techniques require added cooling components.
Further, analog amplifiers have an additional disadvantage when the input signal thereto is taken from a digital source, such as a CD player, or is some other serially digitized format signal. In such cases, an analog amplifier is not fundamentally compatible with serial digital inputs and requires intermediate conversion where the digital signal is first converted to an analog signal either at the source or in the amplifier, such conversion giving rise to added complexity and cost to the amplifier, and often resulting in a degradation of the signal quality.
Binary switching amplifiers have been used by those in the art to achieve higher efficiency than analog amplifiers by the substitution of a switching control operation for the linear control element in the output circuitry of the amplifier. Instead of a linearly biased element, one or more switches are alternated between on and off states in response to a digital command that is time modulated by the amplifier's input analog signal. The time modulated output signal is then filtered to yield an output signal that is an amplified replica of the input signal. Because the active control element comprises one or more switches which are either in an on or an off state, the power loss in the active element can be made relatively low as compared with analog amplifiers.
The low power losses allow substantial benefits in terms of the size and cost of the amplifier as well as in a reduced size and cost of the associated power supplies. Cooling can in most cases be handled by simple thermally conductive paths to the amplifier package itself, additional cooling components often not being required.
The fact that the output circuitry is digitally controlled means that a straightforward interface to digital signal sources is possible. All the processing required to create the time modulated signals can be done at the digital level. For serial data inputs this can be relatively simple and can take advantage of many of the new digital signal processing techniques.
While the use of currently available binary switching amplifiers provide the above discussed advantages over the use of analog amplifiers, it is desirable to improve even further the benefits derived from the use of switching techniques. For example, in such binary switching amplifiers, power dissipation at the output thereof is essentially constant since one or the other of the switching states is always being used. If that power dissipation can be reduced, the overall power efficiency of the amplifier can be increased.
Further, for most applications, e.g., in amplifying music, the average input signal and the resulting desired average output power are small compared to the peak power requirements. The constant power dissipation at the amplifier output, however, is related only to the peak power capacity. Thus, a low signal application must always suffer the power losses associated with the maximum output capabilities.
Similar losses also appear in the amplifier's output filter, which is required in all modulated systems in order to remove the carrier from the switched output. In a binary amplifier, the carrier frequency is the same as the sampling and conversion rate frequency. Since the output switch circuitry is always connected to a power supply which is providing power of one polarity or the other into the load, current is always flowing through the filter elements even if the net output polarity is zero. Since realizable filter elements are not purely reactive, power is dissipated therein, especially in inductive elements, and the power losses in the filter may be as high as those in the switch circuitry itself.
Another problem in using a binary switching amplifier implementation involves the removal of the modulation carrier frequency by the output filter. Because only two output states of opposite polarity are available, small outputs can only be created by cancellation of two large signals of opposite polarity. For a net output of zero, for example, the actual switched output spends equal times in the two opposite states. Such a large square wave signal, when filtered, results in a zero output with some superimposed ripple. Thus, for the creation of a very small output, a very large signal must be supressed by the filter in order to keep the resulting ripple from entering the load. Such operation imposes severe design constraints on the filter both in terms of power handling capacity and in avoiding any nonlinear filter effects which generate spurious harmonic energy at the output.
It should be noted that removal of the carrier in the output is the most difficult when the demanded output is the smallest, at which time, however, the ripple effects are the most noticeable, especially in audio applications. Thus, if the resulting error and the resulting distortion is measured in proportion to the output signal, a binary switching amplifier tends to produce very large percent errors when the signal being amplified becomes small.
It is desirable to design an amplifier which, while taking advantage of the benefits obtained when using switching amplifying techniques as opposed to using analog amplifying techniques, also overcomes the above problems which arise when using a binary switching amplifier system.